original food matter is split into simpler molecules, and the food
is thus changed in its chemical nature. As a result, the compounds
which appear in the decomposing solution are commonly simpler than
the original food molecules. Such products are in general called
decomposition products, or sometimes cleavage products. Sometimes,
however, the bacteria have, in addition to their power of pulling
their food to pieces, a further power of building other compounds
out of the fragments, thus building up as well as pulling down.
But, however they do it, bacteria when growing in any food
material have the power of giving rise to numerous products which
did not exist in the food mass before. Because of their
extraordinary powers of reproduction they are capable of producing
these changes very rapidly and can give rise in a short time to
large amounts of the peculiar products of their growth.

It is to these powers of producing chemical changes in their food
that bacteria owe all their importance in the world. Their power
of chemically destroying the food products is in itself of no
little importance, but the products which arise as the result of
this series of chemical changes are of an importance in the world
which we are only just beginning to appreciate. In our attempt to
outline the agency which bacteria play in our industries and in
natural processes as well, we shall notice that they are sometimes
of value simply for their power of producing decomposition; but
their greatest value lies in the fact that they are important
agents because of the products of their life.

We may notice, in the first place, that in the arts there are
several industries which may properly be classed together as
maceration industries, all of which are based upon the